Bought a few autos. Am I using these rights?

I have a bagseed plant in flowering atm, and I bought some autos to grow while I finish off this bagseed in flowering (12 12). When the bagseed finishes, I'll put fems in there with the auto and increase the light to 18 on 6 off, and then harvest the auto while veging the fems. Am I doing this right?

If you plan on vegging the fems for the entire auto grow then sure. But starting the autos under 12/12 while the bagseed finishes is going to affect your yield in the end.

How much will it effect it? Will it finish more quickly that way and be really small?

If you want to do this. Expect half your original harvest. Autos do best on 20/4 light so either way you won't get that much. If you want a quick harvest. Go for it

For best results you need to replicate the conditions the breeder used! ie: if the breeder used 24/0 that is what will get you your best results. Plants become acclimated to the climates they are bred in after several generations so if possible find out what the breeder used!